This invention relates to aluminum alloy can stock, and more particularly to aluminum alloy sheet for forming one-piece drawn and ironed can bodies, as well as to a process for making can bodies from such sheet and to the product of that process.
Present-day metal cans as used for beverages such as soft drinks, beer and the like are commonly constituted of a seamless one-piece body (which includes the bottom end and cylindrical side wall of the can) and a top end bearing a ring or other opening device. The body is produced from a blank of cold-rolled aluminum alloy sheet (having a gauge, for example, of about 0.014 inch) by a now-conventional forming technique known as drawing and ironing, which involves drawing the blank into a cup and then passing it through a succession of dies to achieve the desired elongated cylindrical body configuration, with a side wall of reduced thickness relative to the bottom end. The top end is separately produced from another sheet aluminum alloy blank, by different but also conventional forming operations, and is secured around its circumference to the top edge of the side wall of the body to provide a complete can.
The severity of the forming procedure employed in producing a drawn-and-ironed can body as described above, and in particular the reduction in thickness of the can side wall (which must nevertheless be able to withstand the internal and external forces exerted on it in use), as well as the fact that the formed can is usually lacquered in an operation necessitating a strength-reducing exposure to heat, require a special combination of strength, formability, and tool wear properties in the alloy sheet from which the can body is made. Significant among these properties are ultimate tensile strength, yield strength, elongation, and earing. Attainment of the requisite combination of properties is dependent on alloy composition and on the processing conditions used to produce the sheet. At present, a preferred sheet for can body blanks is constituted of the alloy having the Aluminum Association (AA) designation 3004, and is produced from conventionally direct-chill-cast ingot up to 24 inches thick by scalping and homogenizing the ingot, and successively hot rolling and cold rolling to the desired final gauge. Often an anneal treatment is used between the hot and cold rolling operations, with the annealing gauge so selected that the amount of cold reduction to final gauge after annealing is about 85%, thereby to provide can body blanks in H19 (extra hard) temper. On the other hand, it is at present preferred to use a different alloy--AA 5082 or AA 5182--for the top end of the can. Compositions of the three aforementioned alloys are given in the following table:
______________________________________ Range or Maximum (%) AA 3004 AA 5082 AA 5182 ______________________________________ Si 0.30 0.20 0.20 Fe 0.7 0.35 0.35 Cu 0.25 0.15 0.15 Mn 1.0-1.5 0.15 0.20-0.50 Mg 0.8-1.3 4.0-5.0 4.0-5.0 Cr -- 0.15 0.10 Zn 0.25 0.25 0.25 Ti -- 0.10 0.10 Other elements 0.05/0.15 0.05/0.15 0.05/0.15 (each/total) Al balance balance balance ______________________________________
It will be understood that all composition percentages above and elsewhere herein are expressed as percentages by weight.
For environmental reasons as well as to conserve materials and energy, used beverage cans are advantageously recycled, i.e. collected as scrap and, after removal of their lacquer or other coatings, melted for recovery and reuse of their metal. In recycling two-alloy cans of the type described above, however, it is not practicable to separate the top ends from the one-piece bodies; hence the recovered metal is a mixture of the end and body alloys, differing in composition from either of those alloys. Attempts to adjust the recovered metal composition, for example, to obtain AA 3004 alloy for making new can bodies, have been uneconomical. U.S. Pat. No. 3,930,395, describing a recovered metal composition (from cans with AA 3004 bodies and AA 5182 ends) that contains 1.8% Mg and 0.8% Mn, proposes adjustment of this composition by addition of manganese or blending with "virgin metal" to produce a 1% Mg, 2-2.5% Mn, alloy for cans. An alloy containing 0.7-1.0% Mn and 1.6-2.0% Mg (with a combined Mn+Mg content of at least about 2.7% ) has been used commercially for farm roofing sheet, but at gauges and with properties not suitable for direct forming of one-piece can bodies. It would be desirable to provide a single alloy composition that could be used for both can ends and one-piece bodies, to facilitate reuse of recovered scrap metal in cans; but none of the aforementioned alloys currently separately employed for ends and bodies have been found satisfactory for such combined use.
It would also be desirable to utilize, e.g. in the manufacture of can body stock, so-called continuous strip casting techniques in place of conventional direct-chill casting of relatively thick ingots. Continuous strip casting is performed by supplying molten metal to a cavity defined between chilled, moving casting surfaces such as parallel runs of a pair of endless belts, thereby to produce a thin (typically less than one inch thick) continuous cast strip. Belt casting apparatus for such casting of strip is described, for example, in U.S. Pat. Nos. 4,061,177 and 4,061,178, the disclosures of which are incorporated herein by this reference. Advantages of continuous strip casting (as compared with direct chill casting of thick ingots) for production of sheet aluminum alloy products include enhanced efficiency and economy, especially in that the thinness of the as-cast strip significantly lessens the extent to which the cast body must be reduced by rolling to a desired sheet gauge, and also in that pre-rolling heat treatment of the as-cast body is simplified or even entirely obviated. Heretofore, however, it has not been feasible to produce sheet for one-piece can bodies from belt-cast strip because AA 3004 alloy rolled from such strip to provide sheet of can body stock gauge at H19 temper does not possess satisfactory properties for commercial drawing and ironing into one-piece can bodies, owing to differences in work-hardening rate, earing, and required annealing temperature between strip-cast and direct chill-cast AA 3004 products.